Field of the Invention
The technical field of this disclosure is Micro Electro Mechanical Systems (MEMS), particularly, MEMS scanning micromirrors.
Related Art
MEMS scanning micromirrors have been developed for the display of visual information. The MEMS scanning micromirror oscillates in one or two dimensions and a laser or other light beam reflects from the mirror surface. Varying the angle and timing of the beam incident on the mirror surface generates a visual image on a screen or other surface, such as a two dimensional display matrix. Different numbers of MEMS scanning micromirrors and lasers are used to produce images of different detail and colors. Exemplary uses for the MEMS scanning micromirrors are video projection (e.g. in head up displays for automotive applications or for pico-projection in mobile phones), optical coherence tomography, and laser Doppler vibrometry.
The first, most well-known category of MEMS scanning micromirrors of which an example is shown in FIG. 1 include a mirror 52 on a mirror body 50 attached to a mirror frame 60 by two collinear torsion beams 41, which create a scanning axis 58 about which the mirror body 50 rotates. The torsion beams 41 both support the mirror body 50 and provide the required torsional stiffness during rotation. The torsion beams 41 are the only points of attachment between the mirror body 50 and the mirror frame 60, and determine the resonant frequency of the MEMS scanning micromirror 30. If this category of mirrors is to be driven resonantly at a very high natural frequency, then the rotational stiffness of the torsion beams 41 should be as high as possible, whereas the rotational inertia of the mirror body 50 should be as low as possible. Increasing the rotational stiffness is difficult because it will also increase the mechanical stresses and increase the risk of failure, unless the tilting angle is reduced. Reducing the inertia of the mirror body 50 can be achieved by reducing the thickness of the mirror, but this will inevitably increase both the static and dynamic deformation of the mirror 50. This will again reduce the optical quality.
Problems with image quality in the present generation of MEMS scanning micromirrors arise due to both static and dynamic mirror deformation. This deformation should not be higher than ±λ/10, where λ is the shortest laser wavelength used in the scanning application. Dynamic deformation is caused by acceleration forces, which are high because of the required optical performance. One of the parameters is the image resolution, which should be high to get a good image quality or to enable large displays. For a perfectly flat mirror, the image resolution is proportional to the product of scanning angle and micromirror diameter. When the image resolution is to be increased then a larger scanning angle or a larger mirror diameter is required, or both. Commonly, also a higher frequency is desired. Larger scanning angles, larger mirror diameters, and higher frequencies all lead to higher acceleration forces. These acceleration forces act on the complete mirror body, but are most prevalent at the largest distance from the rotation axis, which is at the rim of the mirror body. These acceleration forces will dynamically deform the mirror body.
The torque exerted by the torsion-beams, or by any other springs attached to the mirror body such as cantilever beams, leads to the resonant oscillation of the mirror body. This torque also leads to the aforementioned acceleration forces, which are most prevalent at the rim of the mirror. Therefore the torque must be transferred from the attachment point of the torsion-beams or cantilever beams to the tips of the mirror. In between the attachment points and the tips a bending moment will result, which will deform the mirror body, and which will lead to a dynamic deformation of the mirror and the mirror body. Typical deformation patterns in the micromirror surface are S-shaped in cross-sections perpendicular to the micromirror's rotation axis and are arc- or saddle-like parallel to the rotation axis with opposite direction in cross-sections on both sides of the rotation axis. In cross-sections parallel to the rotation axis, the largest deviation from linearity due to deformation appears between their endpoints and midpoints and it is greatest at the micromirror sides parallel to the rotation axis.
Static deformation can be caused by intrinsic mechanical stresses of the mirror or mirror body, for example due to stresses induced by deposition of reflective layers, by processing or removing layers, by temperature gradients or by differences in thermal expansion coefficient of different materials. The static deformation can be reduced by either reducing the stress level or by increasing the stiffness of the mirror body, making it more resilient against the mechanical stress. Increasing the stiffness however requires adding material (and therefore adding mass) or moving material to different locations, both of which will affect the moment of inertia and the resonance frequency.
One approach to reduce both the dynamic and static deformation of the micromirror has been to make the mirror plate thicker to better resist bending moments. Unfortunately, the greater micromirror mass and greater mass moment of inertia requires higher suspension stiffness to maintain a certain resonance frequency, which leads to increased stress in the torsion beams, cantilever beams, and relief springs. Stress in these elements is a limiting factor in achieving higher oscillation frequencies, higher scanning resolution and tilting angle, and longer lifetime and higher reliability. The extra mass in the in the system also reduces the out-of-plain mode resonance frequencies of the micromirror and makes the MEMS mirror more vulnerable to parasitic oscillations due to imperfections and external disturbances, which reduces the performance and image quality as well. Therefore, the increased thickness and mass moment of inertia further limits the achievable image quality.
Another approach to reduce micromirror dynamic deformation has been to increase thickness, and combining this with a removal of material from the back of the mirror body 20 beneath the mirror to reduce mass. FIG. 2 is a bottom perspective view of a mirror body with a mirror frame having a pattern of diamond-shapes. The mirror body 20 with a rotation axis 22 forms diamond shaped stiffeners 26 perpendicular to the rotation axis 22 through the removal of triangular cutouts 28. The diamond shaped stiffeners 26 act as a straight beam transverse to the rotation axis 22, with the most mass near the rotation axis 22 for stiffness and the least mass away from the rotation axis 22 to reduce the mass moment of inertia. The width of the last diamond shaped stiffener 26 is large near the rotation axis 22 defined by the torsion beam 29 since the operating stress is high in this area. Another system of material removal forms a uniform honeycomb shaped pattern across the back of the mirror plate. While the diamond and honeycomb shaped patterns reduce the mass moment of inertia and therewith also the forces acting on the mirror body 20, they reduce the torsional stiffness of the mirror body 20 along the rotation axis 22 as well, which leads to insufficient reduction of the micromirror dynamic deformation. They fail to optimally couple the points of the micromirror subject to the most deformation with the points where the torque is introduced. It is also noted that the mass moment of inertia is not reduced very strongly for the first eigenmode. It is reduced even less strongly for the higher order eigenmodes, such as translation perpendicular to the mirror surface or rotation perpendicular to the rotation axis, than it has been for the oscillation around the rotation axis.
NEE J T ET AL: “Lightweight, optically flat micromirrors for fast beam steering”, OPTICAL MEMS, 2000 IEEE/LEOS INTERNATIONAL CONFERENCE ON 21-24 Aug. 2000, PISCATAWAY, N.J., USA, IEEE, 2000-08-21, pp. 9-10, ISBN: 978-0-7803-6257-4 relates to a tensile-optical-surface (TOS) micromirror, consisting of a tensile polysilicon membrane stretched across a stiff, single-crystal silicon-rib structure that is suspended by torsion-hinges. The torsion-hinges allow the mirror to rotate along a single axis defined by these hinges. It is considered therein that the combined advantages of lightweight, stiff micromirrors provided by TOS (as compared to slab type micro-mirrors) and the high-force actuation of STEC enable large-angle, low-voltage beam steering.
It is noted that EP 2 100 848 discloses a different type of micro-mirror having a mirror plate that is tiltable according to several axes. To that end the mirror plate is suspended with a pair of horizontal springs in a stiffening frame that on its turn is coupled to the main frame with a vertical spring. Each horizontal spring is provided with a pair of spring arms which form a fork. The spring arms are coupled to a respective coupling element mounted at the mirror plate at mutually opposite sides of the rotation axis defined by the horizontal springs.